The need to develop improved soil sampling techniques for horizontally drilled boreholes has become apparent by the increasing use of horizontal drilling to characterize soil at contaminated sites and on linear projects such as tunnels. Horizontal boreholes are presently used for installing utility lines, such as gas lines, electrical or communications conduit and the like. When using horizontal boreholes to characterize sites they provide some obvious advantages over vertical drilling. With vertical drilling, the drilling rig must be positioned directly above the location from which samples are to be taken. With horizontal drilling samples can be taken by extending a borehole horizontally underneath rivers, structures, highways, or environmentally sensitive areas. In addition, vertical drilling is associated with the risk of penetrating impermeable layers, potentially causing crosscontamination between aquifers. This risk can be avoided by horizontal drilling technology.
There are two soil samplers presently in use in conjunction with horizontal directional drilling. One soil sampler is being produced under the Trademark PunchMaster 2000 Core Barrel, by Eastman Christensen Environmental Systems corporation. This soil sampler c onsists of an inner barrel which is encased in an outer tube. The sampler works on a principal similar to a split-spoon or a Shelby Tube core sampler. First a horizontal borehole is drilled up to the target area. The drill string is than withdrawn from the borehole and the boring head is replaced with the sampling tool. The PunchMaster 2000.TM. is advanced into the borehole to the target area while the load on the outer tube is kept constant with an applied hydraulic pressure. At a predetermined location an inner tube is accelerated into the formation by hydraulic pressure. The sample is then drawn back into the outer tube while pressure on the outer tube is maintained to prevent drilling media from contaminating the sample, and the PunchMaster is brought to the surface. This process is repeated for each sample. Another soil sampler is being produced under the by DitchWitch Environmental Systems corporation, located in Perry Okla. This soil sampler consists of a long metal tube with a spring loaded cone-shape cap. A pilot bore is drilled to a distance of approximately 0.3-0.6 of a meter (1 to 2 ft) from the target area. The drill string is then retracted, the cutting head removed, and a soil sampler is connected to the end of the drill string. The sampler is pushed through the bore, then continued to be pushed through the undisturbed soil until the target area is reached. The drill string is retracted approximately 0.46 of a meter (18 inches), and the sampler tube is automatically locked in open position. The sampler is pushed forward 0.3 to 0.6 of a meter (1 to 2 ft), filling the tube with soil. The sampler and drill string are then removed from the bore. The sampling tube is removed and replaced with the drilling head, and the process is repeated.
One disadvantage of both the PUNCHMASTER 2000.TM. and the DITCHWITCH.TM. soil samplers is that the sample must be collected ahead of the drilling bit. To facilitate this the drill string is withdrawn from the borehole and the drill bit is removed in order to attach the soil sampler. A sample is then taken, the drill string is withdrawn from the borehole and the soil sampler is recovered, then the drill bit is reattached in order to drill to the next target location. This requires the entire length of the drill string to be removed from the borehole twice for every sample that is taken. In addition, for contaminated site assessment the soil sampler must be de-contaminated between successive samples to avoid cross-contamination.